Method for drying wood

ABSTRACT

An improvement in the drying of wood which allows drying at an equal or faster than conventional rate with less degrade, fewer stresses, and less total shrinkage, comprising treating the wood prior to drying with a material capable of reducing the interfacial tension at the water-air boundaries in the cell structure of the wood. Preferably the wood is treated with an aqueous solution of a non-volatile surface active agent, such as an alkylarylsulfonate.

U Umted States Patent 1151 3,675,336 Dedrick 1451 July 11, 1972 54] METHOD FOR DRYING WOOD 2,838,564 6/1958 Norwood ..260/$05 A 987,888 3/l9ll Koenman ..34/9.5 [72] mnsmw' wash 2,500,954 3/1950 Loughborough 34/95 [73] Asignee: Weyerlneuser Company, Tacoma, Wash.

. Primar Eraminer-Frederick L. Matteson [22] Fned' July 1969 Asrismnl Examiner-Hany B, Ramey [2|] Appl. No.: 847,511 AnorneyLeslie G. Noller, John M. Crawford and Kenneth W. Vernon Related US. Application Data [63] Continuation of Ser. No. 690,055, Dec. 13, I967. 1 ABSTRACT An improvement in the drying of wood which allows drying at [52] U.S.Ill an equal or faster man conventional mm ass degrade 2; 5 l3 4 8 fewer stresses, and less total shrinkage, comprising treating l the wood prior to drying with a material capable of reducing I the interfacial tension at the water-air boundaries in the cell [56] Re Cited structure of the wood. Preferably the wood is treated with an UNfl-ED STATES PATENTS aqueous solution of a non-volatile surface active agent, such as an alkylarylsulfonate. 2,l06,468 l/l938 SChrOlh.......,......................34/9.5 UX 2,706,342 4/l955 Willis ..34/9.5 lClalm, NoDrawlngs METHOD FOR DRYING WOOD This application is a continuation of application, Ser. No. 690,055, filed Dec. 13,1967.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved process of drying wood wherein the wood is treated prior to drying.

2. Prior Art Relating to the Invention Reducing the moisture content of wood, and particularly wood having moisture contents of or near that of freshly felled wood, in a relatively fast, economical manner, without substantially affecting the properties thereof, has been dealt with extensively. Elaborate drying schemes have been proposed for drying varying species of wood without damaging shrinkage, surface checking, warping, or cupping. Many woods, such as redwood, Wester red cedar, and Hemlock, when dried by conventional kilndrying schemes, develop excessive and irregular shrinkage known as "collapse" or end up with a highly nonuniform moisture content known as wet pockets." Many experts feel that as the free water leaves the cell faces of the wood cells, it exerts a tensile stress of cohesion equal to many atmospheres. These stresses are often great enough to draw the cell wall together, the net effect of which is to collapse the cell or cells, causing a "washboard or corrugated effect on the surfaces of the wood being dried. In less severe situations, when the effect is limited to elements of the cell wall or the pits, the permeability of the structure to movement of vapor may be seriously reduced.

Several ways of avoiding this problem have been propounded, such as substitution of the water contained in the wood cells with a low surface tension organic liquid. These techniques have been designated usually as solvent seasoning. Other chemical means have also been explored as, for example, that disclosed in US. Pat. No. 2,500,954, wherein the surfaces of the wood to be dried are impregnated with a material that generates a gas within the cellular structure of the wood at the temperature of the drying operation. These methods, however, require a large quantity of chemicals, specialized equipment and often present a very high fire hazard.

The process as herein described is relatively inexpensive and can be used in conjunction with conventional kiln-drying techniques. By the process described, wood can be dried at faster rates with less degrade, fewer stresses, smaller shell-to core moisture gradients, less shrinkage, fewer wet pockets, and less energy per unit mass of moisture removed.

SUMMARY OF THE INVENTION This invention relates to an improvement in the drying of wood comprising treating the wood to be dried with a material capable of reducing the interfacial tension of the water-air boundaries within the cell structure of the wood prior to subjecting the wood to drying by conventional or other convective heating methods. Preferably the material used to treat the wood is a surface-active agent or agents in aqueous solution giving a water-air interfacial tension of 30 ergs/cm, or less.

DETAILED DESCRIPTION OF THE INVENTION The process of this invention can be most easily carried out by spraying, dipping, brushing, etc., an aqueous solution of a surface-active agent on the surfaces of the wood to be dried. When only the surfaces of the wood are treated, the surfactant does not penetrate very deeply into the wood. This is probably because wood does not have a continuous water phase which is necessary for migration of the surfactant. By suitable known techniques, however, the wood can be treated, if desired, so that the surfactant or surfactants will be substantially completely diffused throughout the wood although this is not ordinarily necessary. It is thought that the aqueous solution containing the surfactant, when used to treat wood surfaces, penetrates a short distance into the wood surfaces and fills parts of the lumens and particularly the pits connecting lumens with adjacent lumens, and lumens with transverse ray cells with the low surface tension liquid. The pit openings and cell passages in the wood are pulled shut by forces generally present during drying of the wood, with water evaporating from the surfaces of the wood when surfactants are not present. Surface stresses are reduced during the early stages of drying by the action of surfactants and, because more of the cell passages are open, drying proceeds at a faster rate with reduced structural degrade.

The particular surface-active material or composition used to treat the wood prior to drying does not appear to be critical as long as it is capable of reducing the interfacial tension of the water-air boundaries within the wood treated. Preferably, aqueous solutions of non-volatile surfactants giving in aqueous solution a water-air interfacial tension of 30 ergs/cm or less are used. The most common surface-active materials which can be used include sodium alkylarylsulfonates, sold under the trade names ALCONOX by Alconox, lnc.; ALKANOLS by E. l. duPont de Nemours and Co., Inc.; ARTIC SYNTEX I-ID by Colgate-Palmolive Co., alcohol such as sodium lauryl sulfate; alkyl sulfonates wherein the alkyl group contains from l2 to 18 carbon atoms; alkyl phenol polyglycol ether carboxylic acids having the formula wherein n is [0 to 20 and x is an integer ranging from 1 to 5; and other known surface-active agents. The concentration of surfactant in the aqueous solution used to treat the wood surfaces is not critical. A concentration ranging upward from about 0.01 weight percent surfactant can be used.

The wood, after treatment with the surfactant, can be dried by conventional kiln-drying techniques. Best results are obtained if the wood is subjected to treatment with the surfaceactive agent or agents as soon as possible after exposure of a freshly cut surface. By doing so, reduction of stress and degrade which take place during yard storage, and/or yard drying, is accomplished.

Various species, as well as various configurations of wood, can be dried by the process herein described. Wood species such as redwood, Douglas fir, Western red cedar, Southern pine, hemlock, etc., can all be dried by techniques herein described. In addition, wood in the form of lumber, veneer, boards, etc., can be dried by these techniques.

The following examples are given as merely exemplary of the invention, and are not considered to be limiting in any manner.

EXAMPLE I A series of 2" X l0" Douglas fir boards were dipped in a 0.04 weight percent solid solution of a linear alkyl aryl sulfonate surface-active agent sold under the trade name "MEL- O by Klix Chemical Company. The boards were left in the solution for only 5-l0 seconds. The boards were then removed and stacked in a conventional stickered configuration together with an identical number of control boards which had been dipped in water only. The control and surface treated boards were laid side by side in the kiln in order to make drying conditions as much alike as possible. The load was 8 ft. wide and separated by A inch stickers. The treated and control boards were placed side by side.

The boards were then subjected to drying using an entering air temperature adjusted to give a greater than 25 F. drop across the load. The entering air, at a velocity of about 300 fpm, had a wet bulb depression of 30 F. ranging up to F. Drying time was 36 hours.

In Table l the values represent average figures for the 96 boards dried under the previously mentioned conditions.

TABLEI 2" X l" Douglas Fir Kiln Green dry Shell mois mois mois- Core lure Oven Kiln ture ture mois- Green oondry dry conconlure wt. tent wt. wt. tent tent content Controlboards 39.578 43.90 27.585 30.479 I045 6.65 14.25 Surfactant treatedhoards 40.06] 43.32

EXAMPLE ll Further experiments were carried out under the same conditions as described in Example I, using, however, 2" X hemlock and a drying time of 48 hours. The results are reported in Table ll.

By reference to these two examples, it will be noted that (I moisture content of the treated boards averaged lower than the untreated boards, (2) core moisture contents of the treated boards are lower than for the control boards indicating less tendency toward formation ofwet pockets," (3 shell-tocore moisture content gradients averaged less for treated than control specimens, indicating less shell-to-core stress, and (4) severity of degrade was statistically less for the treated than for the control specimens.

Utilization of the process of the instant invention provides potentially large economic advantages. Material cost is low and treating cost is lowv The invention results in a shorter process time, greater moisture uniformity, less redrying, and particularly, less degrade.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

I. A process of drying wood at a faster rate with less degrade, fewer stresses and less total shrinkage by in situ vaporization of water and migration of the vapor outwardly through the surface regions of the wood comprising:

a. applying to the wood, prior to drying, a material capable of reducing the interfacial tension at the water-air boundaries within the cell structure of the wood, and

b. drying said wood in the presence of said material within an appropriate drying apparatus, and wherein said sur' face-active agent comprises an alkylarylsulfonate. 

